Photoconductive elements, also called photoreceptors, are composed of a conducting support and at least one photoconductive layer which is insulating in the dark but which becomes conductive upon exposure to actinic radiation. To form images, the surface of the element is electrostatically uniformly charged in the dark and then exposed to a pattern of actinic radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface and dissipate the surface charge in such areas. The resulting charge pattern on the surface is referred to as an electrostatic latent image. The latent image can be made visible by application of a liquid or dry developer containing finely divided charged toner particles which, if desired, can be transferred and fixed to another surface such as a sheet of paper.
Numerous photoconductive materials have been described as being useful in electrophotography. These include inorganic substances, such as selenium and zinc oxide, and organic compounds, both monomeric and polymeric, such as arylamines, arylmethanes, carbazoles, pyrroles, phthalocyanines and the like. Especially useful are aggregate photoconductive compositions that have a continuous electrically insulating polymer phase containing a finely divided, particulate co-crystalline complex of at least one pyrylium-type dye salt and at least one polymer having an alkylidenediarylene group in a recurring unit.
Aggregate compositions used in photoreceptors can be prepared by several techniques, such as, for example, the "dye first" technique described in Gramza et al., U.S. Pat. No. 3,615,396. Alternatively, they can be prepared by the "shearing" method described in Gramza, U.S. Pat. No. 3,615,415. This latter method involves the high speed shearing of the photoconductive composition prior to coating and thus eliminates subsequent solvent treatment, as was disclosed in Light, U.S. Pat. No. 3,615,414. By whatever method prepared, the aggregate composition is applied with a suitable liquid coating vehicle onto a support or underlying layer to form a separately identifiable multiphase aggregate composition, the heterogeneous nature of which is generally apparent when viewed under magnification, although such compositions may appear to be uniform to the naked eye in the absence of magnification. There can, of course, be macroscopic heterogeneity. Suitably, the pyrylium type dye-salt-containing aggregate in the discontinuous phase is finely-divided, i.e., typically predominantly in the size range of from about 0.01 to about 25 .mu.m.
Photoconductive elements can comprise single or multiple active layers. In a single layer photographic element charge generation and charge transport take place within the same layer. Single active layer aggregate photoconductive elements are described in Light, U.S. Pat. No. 3,615,414 and in Gramza et al., U.S. Pat. Nos. 3,732,180 and 3,615,415. Contois and Rossi, U.S. Pat. Nos. 3,873,312 and 3,873,311, describe the use of aggregate photoconductive compositions and elements containing organic photoconductors with a styrylamino structure. Berwick et al., U.S. Pat. No. 4,175,960, describe a multi-active photoconductive element having an aggregate charge generation layer.
Single active layer aggregate photoconductive compositions have found many commercial applications. They are easily and inexpensively manufactured and are especially suited for use in a drum format. They have the additional advantage of being able to photoconduct to either a negatively or positively charged surface.
A property of aggregate photoconductive compositions that is disadvantageous under certain circumstances is that, when employed as single active layer photoconductors, the layers characteristically have a rough surface. The demand for increasingly higher resolution requires toners with very fine particles. Residual toner, especially that of fine particle size, as well as other materials such as paper powder, accumulate on the rough aggregate layer surface, causing scumming after repeated use. This in turn results in images of poor quality.
This problem of scumming caused by the surface roughness of single active layer aggregate photoconductors can be overcome by the use of a charge-transport surface layer coated over the rough aggregate photoconductive layer. However such multiple active layer aggregate photoconductors are more difficult and expensive to manufacture than single active layer photoconductors.
U.S. Pat. No. 4,626,487 describes, as a solution to the problem of scumming, a developer which contains inorganic fine particles that scrape off residual toner as well as other materials from the photoconductor surface. However, because the aggregate layers containing organic charge transport agents are relatively soft, they are highly susceptible to scratching by the hard inorganic scraper particles. The resulting abrasion damage causes the production of defective copies after relatively low usage. A need exists for a low-cost single active layer photoconductor that is resistant to scumming and that, preferably, is abrasion-resistant.